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Usage of Resistors |
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Pull-up Resistors / Pull-down Resistors / Damping
Resistors |
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Digital circuits ordinary have bus line configuration
on printed circuit boards. Bus lines might be better stabilized by pulling
up to Vcc using resistors around 1k-100 k , or by using termination resistors
matched to signal line impedance. Or, sometimes, damping resistors of around
10-30 are added in series to signal lines to eliminate reflected waveforms.
Network resistors are best fit to these purposes. In case of 8-bit pull-up
resistors/termination resistors, if footprint area needed for 8 chip resistors(1608
size) is considered as 100%, area requirement is only 80% for 2 chip network
resistors CN1J4 (with 4 elements)
and 45% for one CND1J10 (with
bussed 8 elements). In case of 8-bit damping resistors, footprint area requirement
of one CN1F8K is only 60%
of 8 chip resistors(1608 size). |
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Pull-up Resistors / Pull-down Resistors
/ Termination Resistors |
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Damping Resistors |
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Resistors to terminate digital circuits |
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For the high frequency digital circuits, waveforms
are distorted due to the reflection if the terminal impedance (e.g. input impedance
of IC) mismatches with the specific impedance of the transmission line (e.g.
signal line pattern on the printed circuit board). Once the reflection occurs,
the distortion varies from point to point. Therefore it is important to pay
attention to such signal lines as the clock lines have so many branches.
Termination to avoid reflection is an analogous technology, but it is today
an essential technology for high speed digital circuits which should be considered
as distributed constant circuits. Bus lines to connect between printed circuit
boards are terminated with matched impedance from long ago.
Resistors to prevent reflection should be exactly the value of the transmission
lines. Recommended resistors are RK73H,
which are precise enough and excellent in high frequency characteristics. |
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Resistors to terminate differential transmission
line |
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A typical differential transmission scheme
is shown below. There are two drivers to transmit analog or digital signals,
while one side is positive, the other side goes negative symmetrically. This
is an effective way to reject noise, and therefore the signal level can be
low.
This is thus an excellent communication system, signal source impedance Rs+,
Rs-, specific transmission line impedance Zo, input impedance Ri+, and Ri-
are requested to be matched good enough to work correctly. On the contrary,
if the circuit is not electrically symmetrical enough, the system does not
work well, and it even becomes a source of common mode noise. Thus, resistors
to terminate high frequency transmission lines have effects on signal quality.
Resistors of RK73H
types are recommended. |
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Application Notes
Brush up your circuit
Basic Knowledge
